21. Is it possible for a skydiver who jumps second from a plane to put himself in an aerodynamic position and overtake a person who jumped first?

Yes. When you skydive, your velocity doesn't increase indefinitely because the upward force of air resistance eventually balances the downward force of gravity. At that point, you reach a constant velocity (called "terminal velocity"). Just how large this terminal velocity is depends on your shape. It is possible to increase your terminal velocity by rolling yourself into a very compact form. In that case, you can overtake a person below you who is in a less compact form.

No, forces generally depend on the distances between objects, so that two objects that are moving together or apart will experience different amounts of force as they move about. As a result, the total amount of force anywhere can change freely. But there are quantities that have fixed totals for the universe. The most important of these so-called "conserved" quantities is energy.

23. Isn't there "some" acceleration at the very start and very end of an elevator ride? Why does one's stomach take a flop when the elevator stops and not when it starts?

Yes, there is acceleration at the start and stop of an elevator ride. As the car starts, it accelerates toward the destination and as the car starts, it accelerates in the opposite direction. Your stomach takes a flop whenever you feel particularly light, as when you are falling or otherwise accelerating downward. As you accelerate downward, your body doesn't have to support your stomach as much as normal and you feel strange. In fact, you feel somewhat weightless. You have this feeling whenever the elevator starts to move downward (and therefore accelerates downward) or stops moving upward (and there accelerates downward).

Mass is the measure of an object's inertia. You have more mass than a book, meaning that you are harder to accelerate than a book. If you and the book were each inside boxes, mounted on wheels, I could quickly determine which box you were in. I would simply push on both boxes and see which one accelerated most easily. That box would contain the book and you would be in the box that's hard to accelerate. Weight, on the other hand, is the amount of force that gravity (usually the earth's gravity) exerts on an object. You weigh more than a book, meaning that the earth pulls downward on you harder than it does on the book. Again, I could figure out which box you were in by weighing the two boxes. You'd be in the heavier box. So mass and weight refer to very different characteristics of objects. They don't even have the same units (mass is measured in kilograms, while weight is measured in newtons. But fortunately, there is a wonderful relationship between mass and weight: an object's weight is exactly proportional to its mass. Because of this relationship, all objects fall at the same rate. Also, you can use a measurement of weight to determine an object's mass. That's what you do when you weigh yourself on a bathroom spring scale; you are trying to determine how much of you there is-your mass-but you are doing it by measuring how hard gravity is pulling on you—your weight.

25. When you pushed the baseball and bowling ball with an equal force, the baseball went farther on the table because it has a smaller mass. If gravity also exerts an equal force on the 2 balls, like your push, then why do they fall at equal speeds?

The answer is that gravity doesn't exert equal forces on the 2 balls! It pulls down harder on the bowling ball than it does on the baseball. Suppose the bowling ball has 10 times the mass of the baseball. Then gravity will also exert 10 times the force on the bowling ball that it exerts on the baseball. The result is that the bowling ball is able to keep up with the baseball! The bowling ball may resist acceleration more than the baseball, but the increased gravitational force the bowling ball experience exactly compensates.

26. When you shoot a bullet straight upward, doesn't it accelerate upward?

When you shoot a bullet upward, is does accelerate as long as it's in the gun. The burning gases push upward on the bullet and it accelerates upward. But as soon as it leaves the gun, it's a falling object, with the only force on it being gravity (and air resistance).

To throw the ball upward, you temporarily push upward on it with a force greater than its weight. The result is that the ball has a net force (the sum of all forces on the ball) that is upward. The ball responds to this upward net force by accelerating upward. You continue to push upward on the ball for a while and then it leaves your hand. By that time, it's traveling upward with a considerable velocity. But once it leaves your hand, it is in free fall. Nothing but gravity is pushing on it—it's carried upward by its own inertia! In fact, it's accelerating downward at 9.8 m/s^2. It rises for a while, but less and less quickly. Eventually it comes to a stop and then it begins to descend.

28. While gravity supposedly makes all objects accelerate at the same rate, feathers do not seem to comply. What factors affect the feather's acceleration, besides air resistance (which should affect all objects equally)?

Actually, air resistance doesn't affect all objects equally. The feather has so much surface area that it pushes strongly on the air through which it moves and the air pushes back. For an object with very little mass and weight, the feather experiences an enormous amount of air resistance and has great difficulty moving through the air. That's why it falls so slowly. If you were to pack a feather into a tiny pellet, it would then fall just about as fast as other objects. Similarly, you fall much more slowly when your parachute is opened because it then interacts with the air much more effectively.

29. Why do objects on earth accelerate downward at the same speed regardless of their mass?

What you mean here is that they accelerate downward at the same rate ("speed" has a particular meaning that isn't so well suited to discussions of acceleration). This fact comes about because, although massive objects are harder to accelerate, they also experience more weight. Thus a huge stone will fall at the same rate as a small rock because the stone will be pulled downward more strongly by gravity and that extra pull will make up for the stone's greater inertia.

30. Why do two objects of unequal mass fall and hit the ground at the same time?

If one object has twice the mass of the other, then it is twice as hard to accelerate. To make it keep pace with the other ball, it must experience twice the force. Fortunately, gravity pulls on it twice as hard (it has twice the weight of the other ball), so in falling, it does keep pace with the other ball. The two fall together. Just for fun, imagine stepping off the high diving board with two friends. The three of you have essentially identical masses and weights and also fall at the same rate. Now imagine that two of you hold hands as you fall. You are now a single object with twice the mass of your other friend. Nonetheless, you still fall at the same rate. So an object with twice the mass of another falls at the same rate as that other object.